Most aircraft include a tail section with a rudder which provides yaw control. The size of the rudder and the power of the actuators needed to move the rudder are typically determined by the magnitude of the yawing moment that must be produced to control the aircraft in the event of loss of power of an engine. That is, the rudder must be able to correct the yaw instability created by the remaining engines. The rudder creates an appropriate yaw force to ensure that the aircraft remains in a suitable orientation. Aircraft control under normal operating conditions typically requires much smaller yawing moments.
Thus a relatively large tail fin is typically adopted in conventional aircraft configurations, the size of the tail fin being determined by the requirement for a relatively large rudder which can provide aircraft control in emergency situations, such as the loss of an engine. However, the large fin causes significant drag and increases aircraft weight throughout flight despite that the full size of the fin is only needed in emergencies.
US patent 2006284022 proposes alternative methods for creating yaw, for example, using a spoiler surface to generate a low pressure area on a fuselage of an aircraft.